The present invention relates to a fuel assembly, and more particularly to a fuel assembly, including fuel spacers, used for a boiling water reactor.
A fuel assembly used for a boiling water reactor has been disclosed, for example, in Japanese Patent Laid-open No. Hei 2-163695. This fuel assembly includes a plurality of fuel rods and two water rods. The fuel rods are arranged in a square lattice array of 9 rowsxc3x979 columns. These fuel rods and water rods, which form a fuel bundle, are held at mutual specific intervals and are kept immovable by a plurality of fuel spacers arranged in the axial direction.
The fuel spacer includes a large number of cylindrical members; one band member; a plurality of loop-shaped springs; and water rod holding members, each being formed into an approximately xcexa9-shape in transverse cross-section, for holding the water rods. The large number of cylindrical members are joined to each other and are bundled. Each cylindrical member is provided at a lattice position at which a fuel rod is to be inserted, and an associated one of the fuel rods is inserted in each cylindrical member. The band member, formed into a square shape, surrounds the outer periphery of the bundle of the large number of cylindrical members.
The band member has projecting members (bath-tubs) which project on the inner peripheral side of a fuel channel box and is brought in contact with the inner surface of the fuel channel box. The loop-shaped spring is provided at a joined portion between a pair of the adjacent cylindrical members for pressingly supporting the fuel rods inserted in the adjacent cylindrical members. In addition, the loop-shaped spring functions to generate pressing forces when the fuel rods are inserted in the adjacent cylindrical members.
The water rod holding member is joined to two of the cylindrical members adjacent to the water rod. In the case where the loop-shaped springs are intended to be arranged for holding the fuel rods in the two cylindrical members adjacent to the water rod, since the mating one paired with each of the two cylindrical members is not present from the viewpoint of arrangement of springs over the fuel spacer, the spring function cannot be achieved. To cope with such an inconvenience, a spring pressing projecting piece and a spring holding projecting piece are provided at a joined portion between the water rod holding member and each of the above two cylindrical members, so that a pressing force necessary for holding the fuel rod is generated by holding the loop-shaped spring using the spring holding projecting piece and pressing it using the spring pressing projecting piece.
On the other hand, to improve nuclear characteristics of a fuel assembly for a boiling water reactor, as described in Japanese Patent Laid-open No. Hei 5-232273, there is a known configuration provided with a plurality of fuel rods including rods (hereinafter, referred to as short-length fuel rods) each having a length shorter than that of each of the remaining ones of the fuel rods. In the fuel assembly disclosed in the above-mentioned document, to improve the controllability of the reactivity due to reduction in the void coefficient, the short-length fuel rods are arranged in the outer peripheral region of the square lattice array and adjacent to the water rods.
The fuel spaces, as described above, are provided at a plurality of positions in the axial direction. As a result, of the fuel spacers used for the above fuel assembly, including the short-length fuel rods, those positioned above the upper ends of the short-length fuel rods have no fuel rod portions at lattice positions associated with the short-length fuel rods. In this regard, there has been already proposed a structure in which, in the fuel spacer positioned above the upper ends of the short-length fuel rods, the cylindrical members located at the lattice positions associated with the short-length fuel rods are removed to reduce the pressure loss.
However, in the fuel assembly disclosed in Japanese Patent Laid-open No. Hei 5-232273 in which, the short-length fuel rods are arranged in the outermost peripheral region of the square lattice array and adjacent to the water rods, if it is intended to simply remove the cylindrical members located at the lattice positions associated with the short-length fuel rods in the fuel spacer positioned above the upper ends of the short-length fuel rods, the following two problems are likely to occur:
(1) Problem in Terms of Strength
In a usual fuel spacer, short-length fuel rods are located in an inner region (not outermost peripheral region) of a square lattice array of fuel rods. As a result, even by removing cylindrical members located at the lattice positions associated with the short-length fuel rods to reduce the pressure loss, the cylindrical members in the outermost region are continuously in contact with a band member which surrounds the outer periphery of the fuel spacer, so that the structural strength of the entire fuel assembly is little reduced.
For example, if an external force is applied to the fuel spacer via a fuel channel box in case of an earthquake or upon handling of the fuel assembly, the load is first transmitted to projecting members provided on the band member. After that, the load is transmitted, via the band member, to the cylindrical members in the outermost peripheral region of the square lattice array joined to the inner side of the band member, and then the force is sequentially transmitted to the cylindrical members arranged on the inner peripheral side of the square lattice array. In the case where the band member and the cylindrical members are substantially continuously arranged in the transmission path of the load, as described above, the joined body of the band member and the cylindrical members exhibits, as one body, an effect of ensuring the strength, to thereby sufficiently ensure the structural strength of the entire fuel spacer.
On the contrary, in the case where the cylindrical members located at the lattice positions associated with the short-length fuel rods in the outer peripheral region are removed, the arrangement of the cylindrical members in the outermost peripheral region of the square lattice array becomes discontinuous at the positions where the cylindrical members are removed. This makes the strength ensuring effect of the joined body, which is composed of a large number of the cylindrical members, insufficient, and thereby reduces the structural strength of the entire fuel spacer.
To minimize such a reduction in strength, for example, Japanese Patent Laid-open No. Hei 6-3473 discloses a fuel spacer having a structure in which eight bath-tubs for mainly receiving forces applied from a fuel channel box are provided on a square-shaped band member. To be more specific, two of the bath-tubs are provided on each side of the band member in such a manner as to face to two of the bath-tubs provided on the opposed side of the band member; and cylindrical members are necessarily provided at all of the lattice positions between the two of the facing bath-tubs to ensure the strength, and the cylindrical members located at the other lattice positions are removed to reduce the pressure loss.
The fuel spacer disclosed in Japanese Patent Laid-open No. Hei 6-3473, however, causes another problem.
That is to say, in the design of a fuel assembly including short-length fuel rods, the arrangement of the short-length fuel rods varies depending on the required nuclear characteristics. However, in the above fuel spacer, the cylindrical members located at all of the lattice positions between the two of the facing bath-tubs cannot be omitted, and accordingly, if the short-length fuel rod is arranged at one of the lattice positions between the two of the facing bath-tubs, it fails to sufficiently reduce the pressure loss. Conversely, in the case of giving precedence to a sufficient reduction in pressure loss, the short-length fuel rods cannot be arranged at all of the lattice positions between two of the facing bathtubs, and correspondingly, the degree of freedom in design of the fuel assembly is limited.
(2) Problem in Terms of Spring Arrangement
In the above fuel assembly in which the short-length fuel rods are arranged in the outermost peripheral region and adjacent to the water rods, if the cylindrical members located at the lattice positions associated with the short-length fuel rods in the fuel spacer positioned above the upper ends of the short-length fuel rods are simply removed, there occurs another problem.
Description of such a problem will be presented by way of example with reference to the fuel assembly shown in FIGS. 2 and 3 (described later). This fuel assembly includes fuel rods located in a square lattice array of 9 rowxc3x979 columns, and two water rods are arranged in a region in which seven of the fuel rods are arrangeable. Further, in the fuel assembly, four of the short-length fuel rods are arranged one at each midpoint of each side of the outermost periphery of the square lattice array, and two of the short-length fuel rods are arranged at two corners of a square lattice array of 3 rowxc3x973 columns at a central portion of the fuel spacer in such a manner as to be adjacent to the water rods.
It is assumed that in the fuel spacer positioned above the upper ends of the short-length fuel rods in the above-described fuel assembly, cells located at lattice positions associated with the short-length fuel rods are simply removed, and, like the prior art manner, only one kind of loop-shaped springs are used for eliminating an increase in the number of parts. In this case, there occurs a requirement to entirely review the arrangement of the loopshaped springs over the fuel spacer. The re-arrangement of the springs is shown as a comparative example in FIG. 24.
Referring to FIG. 24, in a fuel spacer 103, cells 104 in the form of cylindrical members are bundled and joined to each other. A loop-shaped spring 105 for pressingly holding fuel rods 101 is provided at a joined portion between a pair of the adjacent cells 104. At this time, the provision of the cells 104 at six lattice positions 106a, 106b, 106c, 106d, 106e, and 106f associated with the short-length fuel rods 101A is omitted to reduce the pressure loss. As a result of omission of the cells 104, as shown in FIG. 24, the arrangement of the loop-shaped springs 105 is entirely changed from that described in Japanese Patent Laid-open No. Hei 2-163695.
In the re-arrangement of the springs shown in FIG. 24, two of the loop-shaped springs 105A and. 105B, each of which is free, that is, has no mating cell paired therewith, are adjacently present at each of the lattice positions 106e and 106f, that is, at the two corners of the square lattice array of 3 rowxc3x973 columns in the, central portion of the fuel spacer, in such a manner as to be adjacent to the water rods 102. To allow the loop-shaped springs 105A and 105B to exhibit a spring function, it is required to provide a spring pressing structure allowing the loopshaped springs 105A and 105B to simultaneously generate pressing forces at each of the lattice positions 106e and 106f. 
However, such a spring pressing structure for mounting two or more of the free loop-shaped springs at one lattice position has not been known. Further, it is also required to examine a connection mechanism between the spring pressing structure and a water rod holding member 108, formed into an approximately xcexa9-shape in transverse cross-section, for holding the water rod.
A first object of the present invention is to provide a fuel assembly which is capable of sufficiently reducing the pressure loss of a fuel spacer positioned upward from the upper ends of short-length fuel rods irrespective of the arrangement of the short-length fuel rods and ensuring the structural strength of the fuel spacer.
A second object of the present invention is to provide a fuel assembly which is capable of sufficiently reducing the pressure loss of a fuel spacer positioned upward from the upper ends of short-length fuel rods irrespective of the arrangement of the short-length fuel rods and reasonably arranging the necessary minimum number of one kind of loop-shaped springs over the fuel spacer.
(1) To achieve the above first object, according to the present invention, there is provided a fuel assembly including:
a plurality of fuel rods located in a square lattice array, the fuel rods including a plurality of short-length fuel rods each having a fuel active length shorter than that of each of the remaining ones of the fuel rods;
at least one water rod arranged in a region in which one or more of the fuel rods are arrangeable; and
a plurality of fuel spacers, provided at a plurality of positions in the axial direction, for holding the plurality of fuel rods and the at least one water rod with mutual radial intervals thereof kept immovable;
wherein the plurality of short-length fuel rods include at least one first short-length fuel rod arranged in the outermost peripheral region of the square lattice array;
each of the plurality of fuel spacers includes a plurality of cylindrical members which are connected to each other and in which the fuel rods are to be inserted respectively, and a band member for surrounding the outermost peripheries of the plurality of cylindrical members; and
the plurality of fuel spacers include first fuel spacers positioned above the upper end of the at least one first short-length fuel rod, and at least one of the first fuel spacers is configured such that one of the cylindrical members, located at a first lattice position associated with the at least one first short-length fuel rod, is omitted, and, instead, a first supporting member for connecting two first cylindrical members, of the plurality of cylindrical members, adjacently located on both sides of the first lattice position in the outermost peripheral region adjacent to the band member, is provided at the first lattice position.
When a plurality of fuel spacers are provided at a plurality of positions in the axial direction, no fuel rod is present at the first lattice position associated with the at least one first short-length fuel rod in the first fuel spacer positioned above the upper end of the at least one first short-length fuel rod. Accordingly, by omitting the cylindrical member located at the first lattice position, it is possible to reduce the flow resistance of a coolant flowing upward in the fuel assembly, and hence to sufficiently reduce the pressure loss.
The omission of the cylindrical member makes for a discontinuous arrangement of the cylindrical members located in the outermost peripheral region of the square lattice array at the first lattice position. However, by connecting the two first cylindrical members on both sides of the first lattice position in the outermost peripheral region to the band member by means of the first supporting member, the two first cylindrical members are fixed to each other via the band member. With this configuration, a load transmitted from the band member can be received by the connected structure composed of the two first cylindrical members and the first supporting member fixedly connected to each other.
This allows the first fuel spacer to exhibit a structural strength substantially comparable to that of a fuel spacer in which the cylindrical member is present at the first lattice position. The first supporting member can be arranged irrespective of arrangement of the bath-tubs, and accordingly, unlike the prior art structure, even when the first short-length fuel rod is located at a position between the two opposed bath-tubs in the outermost peripheral region, the cylindrical member at the position can be removed and instead the first supporting member can be provided thereat to sufficiently reduce the pressure loss.
As described above, it is possible to sufficiently reduce the pressure loss of the first fuel spacer positioned above the upper end of the first shortlength fuel rod while usually ensuring the structural strength of the first fuel spacer irrespective of the arrangement of the first short-length fuel rod.
(2) To achieve the above first object, according to the present invention, there is also provided a fuel assembly including:
a plurality of fuel rods arranged in a square lattice array, the fuel rods including a plurality of short-length fuel rods each having a fuel active length shorter than that of each of the remaining ones of the fuel rods;
at least one water rod arranged in a region in which one or more of the fuel rods are arrangeable; and
a plurality of fuel spacers, provided at a plurality of positions in the axial direction, for holding the plurality of fuel rods and the at least one water rod with mutual radial intervals thereof kept immovable;
wherein the plurality of short-length fuel rods include at least one first short-length fuel rod arranged in the outermost peripheral region of the square lattice array;
each of the plurality of fuel spacers includes a plurality of cylindrical members which are connected to each other and in which the fuel rods are to be inserted respectively, and a band member for surrounding the outermost peripheries of the plurality of cylindrical members; and
the plurality of fuel spacers include first fuel spacers positioned above the upper end of the at least one first short-length fuel rod, and at least one of the first fuel spacers is configured such that one of the cylindrical members, located at a first lattice position associated with the at least one first short-length fuel rod, is omitted, and, instead, a second supporting member for connecting two pieces of first cylindrical members, of the plurality of cylindrical members, adjacently located on both sides of the first lattice position in the outermost peripheral region, to a second cylindrical member, of the plurality of cylindrical members, located inwardly from and adjacently to the first lattice position.
When a plurality of the fuel spacers are provided at a plurality of positions in the axial direction, no fuel rod is present at the first lattice position associated with the at least one first short-length fuel rod in the first fuel spacer positioned above the upper end of the at least one first short-length fuel rod. Accordingly, by omitting the cylindrical member located at the first lattice position, it is possible to reduce the flow resistance of a coolant flowing upward in the fuel assembly, and hence to sufficiently reduce the pressure loss.
The omission of the cylindrical member makes for a discontinuous arrangement of the cylindrical members located in the outermost peripheral region of the square lattice array at the first lattice position. However, by connecting the two first cylindrical members on both sides of the first lattice position in the outermost peripheral region to the second cylindrical member located inwardly from and adjacently to the first lattice position by means of the second supporting member, the two first cylindrical members are fixed to each other via the second cylindrical member. With this configuration, a load transmitted from the band member can be received by the connected structure composed of the two first cylindrical members, second supporting member, and second cylindrical member fixedly connected to each other.
This allows the first fuel spacer to exhibit a structural strength substantially comparable to that of a fuel spacer in which the cylindrical member is present at the first lattice position. The second supporting member can be arranged irrespective of arrangement of the bath-tubs, and accordingly, unlike the prior art structure, even when the first short-length fuel rod is located at a position between the two opposed bath-tubs in the outermost peripheral region, the cylindrical member at the position can be removed and instead the second supporting member can be provided thereat to sufficiently reduce the pressure loss.
As described above, it is possible to sufficiently reduce the pressure loss of the first fuel spacer positioned above the upper end of the first shortlength fuel rod while usually ensuring the structural strength of the first fuel spacer irrespective of arrangement of the first short-length fuel rod.
(3) In the configuration of the invention described in section (1) or section (2), preferably, the first or second supporting member has a transverse cross-section smaller than that of the cylindrical member.
(4) In the configuration of the invention described in section (1) or section (2), preferably, the at least one first fuel spacer includes, at a connection portion between one of the two first cylindrical members and the first or second supporting member, spring pressing means for imparting a pressing force to a spring for holding the fuel rod inserted in the one of the two first cylindrical members. With this configuration, it is possible to increase the degree of freedom in arrangement of the fuel rod holding springs in the fuel spacer.
(5) In the configuration of the invention described in section (1) or section (2), preferably, the plurality of short-length fuel rods include at least one second short-length fuel rod arranged in a region adjacent to the at least one water rod;
each of the plurality of fuel spacers includes a water rod holding member connected to those of the plurality of cylindrical members arranged in the innermost peripheral region of the square lattice array for holding the at least one water rod; and
the at least one first fuel spacer is configured such that one of the cylindrical members, located at a second lattice position associated with the at least one second short-length fuel rod, is omitted and instead a third supporting member for connecting two third cylindrical members, of the plurality of cylindrical members, located outwardly from and adjacently to the second lattice position in the square lattice array to the water rod holding member.
(6) In the configuration of the invention described in section (1) or section (2), preferably, each of the plurality of fuel spacers includes a plurality of first projecting members provided on the band member, the first projecting member projecting between the two adjacent ones of the cylindrical members in the outermost peripheral region of the square lattice array for introducing the flow of a coolant; and
the at least one first fuel spacer is configured such that at least one of the projecting members adjacent to the first or second supporting member is omitted.
The provision of the first projecting members in each fuel spacer is effective to direct the flow of a coolant in the fuel assembly toward the fuel rod side as much as possible, and hence to improve the effect of cooling the fuel rods and enhance the critical power characteristic. In the first fuel spacer positioned above the upper ends of the short-length fuel rods, no fuel rod is present at the first lattice position at which the first or second supporting member is provided. Accordingly, the provision of the first projecting members in the vicinity of the first or second supporting member is not effective so much. For this reason, by omitting the first projecting members adjacent to the first or second supporting member, it is possible to reduce an increment of pressure loss due to the projecting shapes thereof, and hence to further reduce the pressure loss.
(7) In the configuration of the invention described in section (6), preferably, the at least one first short-length fuel rod is arranged in the outermost peripheral region except for four corners of the square lattice array. The effect of improving the critical power characteristic is largest at the four corners in the outermost peripheral region of the square lattice array, and becomes smaller at other positions in the outermost peripheral region.
Accordingly, in the case where the first or second supporting member is arranged at a lattice position associated with the first short-length fuel rod, other than the four corners in the outermost peripheral region, the effect of improving the critical power characteristic is not reduced so much even by omitting the projecting members adjacent to the supporting member. That is to say, the omission of the projecting members is more effective in the case where the first or second supporting member is arranged at a lattice position associated with the first short-length fuel rod, other than at the four corners in the outermost peripheral region.
(8) To achieve the above second object, according to the present invention, there is provided a fuel assembly including:
a plurality of fuel-rods arranged in a square lattice array, the fuel rods including a plurality of short-length fuel rods each having a fuel active length shorter than that of each of the remaining ones of the fuel rods;
at least one water rod arranged in a region in which one or more of the fuel rods are arrangeable; and
a plurality of fuel spacers, provided at a plurality of positions in the axial direction, for holding the plurality of fuel rods and the at least one water rod with mutual radial intervals thereof kept immovable;
wherein the plurality of short-length fuel rods include at least one first short-length fuel rod arranged in the outermost peripheral region of the square lattice array, and at least one second short-length fuel rod arranged at a lattice position adjacent to the at least one water rod;
each of the plurality of fuel spacers includes a plurality of cylindrical members which are connected to each other and in which the fuel rods are to be inserted respectively, and first loop-shaped springs each of which is provided at a joined portion between a pair of the adjacent ones of the plurality of cylindrical members for pressing two of the fuel rods inserted in the adjacent cylindrical members;
the plurality of fuel spacers include first fuel spacers positioned above the upper ends of the first and second short-length fuel rods, and at least one of the first fuel spacers is configured such that those of the plurality of cylindrical members located at first and second lattice positions associated with the first and second short-length fuel rods are omitted;
each of those of the plurality of cylindrical members located at lattice positions adjacent to the second lattice position has on the second lattice position side a second loop-shaped spring for pressing the fuel rod in the cylindrical member; and
the plurality of second loop-shaped springs located at the lattice positions adjacent to the second lattice position are supported by a spring pressing member provided at the second lattice position.
In the at least one first fuel spacer positioned above the upper ends of the first and second shortlength fuel rods, the pressure loss can be reduced by omitting the unnecessary cylindrical members located at the first and second lattice positions.
As a result of removal of the cylindrical members located at the first and second lattice positions in the first fuel spacer, there may be a requirement for reviewing the arrangement of the loop-shaped springs over the fuel spacer. In this case, there may be often provided a second loop-shaped spring of the same kind as that of the first loop-shaped spring on the second lattice side of each of a plurality of the cylindrical members located at lattice positions adjacent to the second lattice position for pressing the fuel rod in the cylindrical member.
Incidentally, the loop-shaped spring is generally configured such that it does not generate any pressing force in the state in which the fuel rods are not inserted in the adjacent cylindrical members between which the loop-shaped spring is mounted. In this regard, each of the plurality of second loop-shaped springs is freely movable on the second lattice position side because no fuel rod is present at the second lattice position, and therefore, it does not generate any pressing force. To cope with such an inconvenience, according to the present invention, the spring pressing member is provided at the second lattice position. The spring pressing member supports the plurality of free second loop-shaped springs in such a manner that the second loop-shaped springs generate pressing forces applied to the associated fuel rods.
(9) In the configuration of the invention described in section (8), preferably, the spring pressing member in the at least one first fuel spacer includes a plurality of spring holding portions which are inserted in the loops of the plurality of second loop-shaped springs for holding the plurality of second loop-shaped springs respectively; and a plurality of spring pressing portions which are brought in contact with the loops of the plurality of second loop shaped springs from the outer peripheral side for supporting the plurality of second loop-shaped springs such that the plurality of second loop-shaped springs generate pressing forces applied to the associated ones of the fuel rods respectively.
(10) In the configuration of the invention described in section (9), preferably, the plurality of spring holding portions include a plurality of spring holding projecting pieces and the plurality of spring pressing portions include a plurality of spring pressing projecting pieces; and
the plurality of spring holding projecting pieces all project in one direction, and at least one of the plurality of spring pressing projecting pieces projects in the opposite direction.
The spring holding projecting pieces and the spring pressing projecting pieces of the spring pressing member are generally formed by cutting the base plate portion of the spring pressing member into tongue shapes. The spring pressing projecting pieces are brought in contact with the loops of the second loop-shaped springs from the outer peripheral side, so that the spring pressing projecting pieces are required to project on the inner side of the spring pressing member more than the spring holding projecting pieces inserted in the loops of the loop-shaped springs. Accordingly, if the projecting direction of all of the spring pressing projecting pieces is made identical to the projecting direction of the spring-holding projecting pieces, the cut-in amount along both the sides of each of the spring pressing projecting pieces must be made larger, and correspondingly, the width of a portion, which is equivalent to the non-cut root portion of the spring pressing projecting piece, of the spring pressing member becomes smaller. This makes it difficult to ensure sufficient strength and rigidity against the pressing forces of the second loop-shaped springs.
To cope with such a problem, according to the present invention, the projecting direction of at least one of the spring pressing projecting pieces is opposed to the projecting direction of the spring holding projecting pieces. This is effective to solve such a problem and to ensure sufficient strength and rigidity.
(11) In the configuration of the invention described in section (9), preferably, the plurality of spring holding portions include a plurality of spring holding projecting pieces and the plurality of spring pressing portions include a plurality of spring pressing projecting pieces; and
at least one of the plurality of spring pressing projecting pieces is configured such that the leading end thereof is connected to a portion, which is opposed to the leading end of the spring pressing projecting piece, of a base plate portion of the spring pressing member.
Since at least one of the spring pressing projecting pieces is configured such that the root portion and leading end thereof are integrated with the base plate portion, it is possible to ensure sufficient strength and rigidity against the pressing forces of the second loop-shaped springs.
(12) In the configuration of the invention described in section (8), preferably, the at least one first fuel spacer further includes a water rod holding member for holding the at least one water rod in the radial direction; and the spring pressing member is joined to the water rod holding member. The joined body composed of the spring pressing member and the water rod holding member can support the second loop-shaped springs so as to be free on the second lattice position side such that the second loop-shaped springs generate pressing forces applied to the fuel rods, and also it can hold the at least one water rod in the radial direction.
(13) In the configuration of the invention described in section (8), preferably, the spring pressing member of the at least one first fuel spacer serves as a water rod holding member for holding the at least one water rod in the radial direction.
(14) In the configuration of the invention described in section (8), preferably, the plurality of second loop-shaped springs are provided on the second lattice sides of two of the plurality of cylindrical members, positioned adjacent to each other on the second lattice in the row direction and column direction.
(15) To achieve the above second object, according to the present invention, there is also provided a fuel assembly including:
a plurality of fuel rods arranged in a square lattice array of 9 rowsxc3x979 columns, the fuel rods including a plurality of short-length fuel rods each having a fuel active length shorter than that of each of the remaining ones of the fuel rods;
two water rods arranged in a region within an array of 3 rowsxc3x973 columns of the square lattice array, in which region seven pieces of the fuel rods are arrangeable; and
a plurality of fuel spacers, provided at a plurality of positions in the axial direction, for holding the plurality of fuel rods and the water rods with mutual radial intervals kept immovable;
wherein the plurality of short-length fuel rods include four first short-length fuel rods each being arranged at the midpoint on each side of a square shape formed by the outermost peripheral region of the square lattice array, and two second short-length fuel rods arranged in a region of the array of 3 rowsxc3x973 columns except for the region in which the two water rods are arranged;
each of the plurality of fuel spacers includes a plurality of cylindrical members which are connected to each other and in which the fuel rods are to be inserted respectively, and first loop-shaped springs each of which is provided at a joined portion between a pair of the adjacent ones of the plurality of cylindrical members for pressing two of the fuel rods inserted in adjacent cylindrical members;
the plurality of fuel spacers include first fuel spacers positioned above the upper ends of the first and second short-length fuel rods, and at least one of the first fuel spacers is configured such that four of the cylindrical members located at four first lattice positions associated with the four first short-length fuel rods and two of the cylindrical members located at two second lattice positions associated with the two second short-length fuel rods are omitted;
each of two of the plurality of cylindrical members, located at lattice positions adjacent to each of the second lattice positions in the row direction and column direction, has on the second lattice position side a second loop-shaped spring for pressing the fuel rod in the cylindrical member; and
the two second loop-shaped springs located at the two lattice positions adjacent to the second lattice position are supported by a spring pressing member provided at the second lattice position.
(16) In the configuration of the invention described in section (15), preferably, the spring pressing member has at least one second projecting member which projects outward from the outer peripheral side of the spring pressing member for introducing the flow of a coolant in the projecting direction of the second projecting member.
The spring pressing member at the second lattice position, which is at a level at which no fuel rod is present, is not required to be cooled. Accordingly, to direct the flow of a coolant passing through the spring pressing member toward the other fuel rods around the lattice position as much as possible, the second projecting members are provided on the spring pressing member. This makes effective use of the coolant and hence improves the effect of cooling the fuel rods.